Joan Boulanger

An Improved Soot Formation Model for 3D Diesel Engine Simulations

In this study, a phenomenological three-equation soot model was developed for modelling soot formation in diesel engine combustion based on considerations of acceptable computational demand and a qualitative description of the main features of the physics of soot formation. The model was developed based on that of Tesner et al. The model was implemented into the commercial STAR-CD CFD package and was demonstrated in the modelling of soot formation in a single-cylinder research version of Caterpillar 3400 series diesel engine with exhaust gas recirculation. Numerical results show that the new soot formulation overcomes most of the drawbacks in the existing soot models and demonstrates a robust and consistent behaviour with experimental observation. Compared to the existing soot models for engine combustion modelling, some distinct features of the new soot model include: no soot is formed at low temperature, minimal model parameter adjustment for application to different fuels, and there is no need to prescribe the soot particle size.Copyright

NEW DEVELOPMENTS IN FREQUENCY DOMAIN OPTICAL TOMOGRAPHY. PART II. APPLICATION WITH A L-BFGS ASSOCIATED TO AN INEXACT LINE SEARCH

This second part deals with the application of the presented formulations for the reconstruction of optical properties in frequency domain optical tomography with the finite element method. We use the Limited memory BFGS algorithm with an inexact line search in order to avoid numerous evaluations of the objective function. Normalization of the objective function with measurements and independent scaling of its gradient are used to improve the quality of the reconstruction. The results show a better recovering of both the absorption and scattering coefficients.

An Improved Phenomenological Soot Formation Submodel for Three-Dimensional Diesel Engine Simulations: Extension to Agglomeration of Particles into Clusters

An extension to a phenomenological submodel for soot formation to include soot agglomeration effects is developed. The improved submodel was incorporated into a commercial computational fluid dynamics code and was used to investigate soot formation in a heavy-duty diesel engine. The results of the numerical simulation show that the soot oxidation process is reduced close to the combustion chamber walls, due to heat loss, such that larger soot particles and clusters are predicted in an annular volume at the end of the combustion cycle. These results are consistent with available in-cylinder experimental data and suggest that the cylinder of a diesel engine must be split into several volumes, each of them with a different role regarding soot formation.

Modulation Frequency Analysis of 1-D Semi-Transparent Medium

Frequency analysis is an interesting alternative to the full transient optical probing due to its relatively simpler frame of investigation. Both frequency analysis and the full transient probing are superior to the steady-state tomography because of more information gathered per measurement point. The study of the systematic response of a semi-transparent slab to a harmonic excitation through a power modulated laser is therefore of interest and is conducted to investigate its optical properties using an accurate algorithm to solve the Radiative Transfer Equation in the frequency domain. It is found that a semi-transparent slab behaves like a low-pass filter for the power modulated light and this behaviour is mainly controlled by the scattering events occurring on the light path (dispersion). The results found are consistent with those in the literature obtained from experiments or simpler analysis models.Copyright

Investigating renewable fuel combustion II: DNS of DME and n‐heptane ignition in a turbulent non‐homogeneous flow with high dissipation

Direct Numerical Simulations (DNS) are conducted to gain insight into the early ignition process in a non‐homogeneous turbulent flow. This is the second part of a study to investigate the characteristics of renewable oxygenated fuel versus conventional diesel. In the first study, Unsteady Reynolds‐Averaged Navier‐Stokes (URANS) simulations of a research diesel engine were carried out. Attention was paid to the impact of the liquid phase properties on the in‐cylinder flow patterns. DNS is employed here to investigate local micro‐mixing effects on ignition. DiMethyl‐Ether (DME) is the model for oxygenated fuel while conventional hydrocarbon (HC) is represented by n‐heptane. Subtle effects of micro‐mixing can modify the combustion phasing in the compression‐ignition mode. For the same temperature and mixture fraction distributions, the location of the stoichiometry iso‐surface is different for the oxygenated and conventional fuels, and as a result ignition kernels develop at a higher temperatures for n‐heptane than for DME. This may change the conclusions with respect to the cetane number and homogeneous ignition investigations.

A Modified Cost Function Based Optical Tomography Reconstruction Algorithm Optimized for Finite-Elements Methods

In optical tomography, the optical properties of the medium under investigation are obtained through the minimization of an objective function. Generally, this function is expressed as a discrete sum of the square of the errors between measurements and predictions at the detectors. This paper introduces a continuous form of the objective function by taking the integral of the errors. The novelty is that the surfaces of the detectors are taken into account in the reconstruction and a compatibility is obtained for all finite element formulations (continuous and discontinuous). Numerical tests are used to compare the reconstructions with both objective functions. It is seen that the integral approach leads to low values of objective functions those reconstructions may be affected by rounding errors. Scaling of the objective function and its gradient shows that both methods give comparable accuracy with an advantage to the continuous approach where the integral acts as a filter of noise.© 2011 ASME

THEORETICAL DEVELOPMENT FOR REFRACTIVE INDEX RECONSTRUCTION FROM A RADIATIVE TRANSFER EQUATION-BASED ALGORITHM

This study is devoted to the mathematics behind a reconstruction methodology based on the radiative transfer equation of a refractive index arbitrary distribution. The targeted algorith m should be of the least-squares and gradient type, r elying on the adjoint to the radiative transfer equ ation for varying refractive index, which is a novelty. P reliminary tests are demonstrated on generic phantoms.

NEW DEVELOPMENTS IN FREQUENCY DOMAIN OPTICAL TOMOGRAPHY. PART I. FORWARD MODEL AND GRADIENT COMPUTATION

This paper deals with a gradient-based frequency domain optical tomography method where the collimated source direction is taken into account in the computation of both the forward and the adjoint models. The forward model is based on the least square finite element method associated to the discrete ordinates method where no empirical stabilization is needed. In this first part of the study, the forward model is highlighted with an easy handling of complex boundary condition through a penalization method. Gradient computation from an adjoint method is developed rigorously in a continuous manner through a Lagrangian formalism for the deduction of the adjoint equation and the gradient of the objective function. The proposed formulation can be easily generalized to stationary and time domain optical tomography by keeping the same expressions.

An Improved Soot Formation Model for 3-D Diesel Engine Simulations

In this study, a phenomenological three-equation soot model was developed for modelling soot formation in diesel engine combustion based on considerations of acceptable computational demand and a qualitative description of the main features of the physics of soot formation. The model was developed based on that of Tesner et al. The model was implemented into the commercial STAR-CD CFD package and was demonstrated in the modelling of soot formation in a single-cylinder research version of Caterpillar 3400 series diesel engine with exhaust gas recirculation. Numerical results show that the new soot formulation overcomes most of the drawbacks in the existing soot models and demonstrates a robust and consistent behaviour with experimental observation. Compared to the existing soot models for engine combustion modelling, some distinct features of the new soot model include: no soot is formed at low temperature, minimal model parameter adjustment for application to different fuels, and there is no need to prescribe the soot particle size.Copyright

An Optical Tomography Reconstruction Algorithm With the Discontinuous Galerkin Method

In optical tomography, the optical properties are recovered through an iterative scheme, which consists in minimizing the errors between the measurements and the predications of a forward model. Traditionally, finite volumes or continuous finite elements formulations of light transport are used as a forward model for the predictions. we have introduced an integral form of the objective function that takes into account the surface of the detectors, and thus making compatible the inverse approach with all finite elements formulations (continuous and discontinuous). This present paper illustrates this novel approach by developing a Discontinuous Galerkin formulation as a forward model. Numerical tests are performed to gauge the accuracy of the method with a gradient-based algorithm. The results show that the reconstruction is accurate and can be affected by noise on the measurements as expected. A filtering of the gradient improves the quality and the accuracy of the reconstruction.Copyright